The present invention is related to a method for controlling the dimensions of an elongated material rolled in a rolling mill comprising at least two mill stands arranged after each other, each of said stands comprising two spaced rolls, said elongated material being fed between the rolls of each stand by rotating the rolls, wherein the material is subjected to stresses in the longitudinal direction thereof during the rolling operation. More particularly, it relates to a rolling mill for the production of materials with shapes different from sheets or strips, such as rods and bars of various types.
The present invention is further related to a device for controlling the dimensions of an elongated material rolled in a rolling mill.
A rolling mill normally comprises a plurality of mill stands arranged after each other. Each of said stands comprises two spaced rolls with parallel rotation axes. A material is fed between the rolls of each stand, and thereby rolled, by rotating the rolls. The rolled material will elongate and spread as the cross-section of the rolled material is reduced as it passes through said stands. The cross section after each stand is defined by the passdesign and the layout of the mill. The cross section is defined by the height and the width of the material leaving a roll gap.
Typically, the rolls of a first stand rolls the material in a first direction, and the rolls of a second, adjacent stand rolls the material in a direction perpendicular to the first direction. Usually, the rolls of said first stand have horizontally directed rotation axes and the rolls of said second stand have vertically directed rotation axes. Thus, a vertical dimension of the rolled material is reduced in said first stand and the horizontal dimension of the rolled material is reduced in said second stand.
As the material passes between the mill stands, there exists a tension, i.e. a tensile or compressive stress, in the longitudinal direction of the material between any two mill stands. A tension in the rolled material between any two stands is described hereafter as an interstand tension. The interstand tension is changed by adjusting the rotational speed of the rolls of a first mill stand relative to the rotational speed of the rolls of a second mill stand.
In the practise of semi-continuous hot rolling, billets are rolled one at a time. The first part of the billet entering the rolling mill is known as the head end. The final part of the rolled material is known as the tail end. When the tail end of the billet leaves one of said stands, the degree of control over the dimensions of the rear portion of the rolled material is reduced. The interstand tension in the rear portion of the rolled material changes as the tail end of the billet leaves the stand. Most frequently, the interstand tension is reduced for the tail. A compressive stress is normally generated in the rear portion, which causes an increase in width at the end of the tail. The rear portion is normally defined as substantially the part of the material extending between two successive stands. The part of the material, in which said increase in width is imposed, is normally useless and wasted after the rolling operation. However, a quantity of rolled material in the rear portion of the rolled material represents a considerable part of the rolled material. Thus, an additional control method is desired for controlling the dimensions of the rear portion of the rolled material in order to reduce the amount of sub-standard material produced in each rolled material.
The object of the invention is to reduce the amount of sub-standard material of a rolled material. A further object of the invention is to design ways to control the dimensions of a rear portion of the material.
These objects are achieved in that a rear portion of said material is subjected to an additional tension substantially in the longitudinal direction of the material when a rear end of the material is in the vicinity of a first stand and has not yet passed said stand. In this way, said additional tension compensates for the generated increase in width in the rear portion of said material when the rear end of the material has passed the first stand. A further advantage is that the control method may be fitted easily and at low capital cost to an existing rolling mill, as the method only requires additional measurement and control equipment. By controlling the dimensions of the rear portion of the rolled material, wear and miss-alignment of the rolls and guide rails are reduced.
According to a preferred embodiment of the invention, the rear portion of said material comprising the rear end of said material is subjected to said additional tension. In this way, the amount of sub-standard material of the rolled material is minimised. Preferably, the rear portion of said material is subjected to said additional tension as said rear end of the material is in contact with the rolls of said first stand.
According to another preferred embodiment of the invention, said material is in contact with the rolls of a second stand during said tension application. Thus, the additional tension may be applied by controlling the rolls of said first and second stand. Preferably, the rear portion of said material is subjected to said additional tension by decreasing the rotational speed of the rolls of said first stand relative to the rotational speed of the rolls of said second stand.
The rear portion of said material is preferably subjected to said additional tension with a first magnitude based partly on width measurements made on at least one elongated material previously fed passed said first stand and partly on width measurements on itself. Thus, the width measurements, and the magnitude of the additional tension used on said previously fed elongated material are used and the first magnitude of the additional tension is calculated based on these parameters. Preferably, said width measurements and the magnitude of the tension are stored for a plurality of previously fed materials, and used for the calculation of the first magnitude of the tension of the actual, subsequent rolled material.
The present method is applicable to any part of the rolling mill for controlling the width of the rear portion of a rolled material.
The inventive device for controlling a rolling mill is more closely defined in the claims and the following description.